THE PROBLEM OF THE ANORTHOSITES 105 



gravities. But if we deduce from this that the gravitational dis- 

 tribution of crystals and crystal hzation-differentiation solves the 

 whole problem of the anorthosites, we will meet with difficulties in 

 the course of the crystallization, and especially in the fact that 

 monomineral rocks represent relatively rare facies, while normal 

 undifferentiated gabbro-noritic rocks are by far more frequent. 

 Why, we must ask, does crystallization-differentiation not always 

 lead to these final products of gravitative liquation? Bowen's ex- 

 perimental researches and his deductions from them, give induc- 

 tions which in many points are theoretically correct for a theory of 

 the crystalhzation of a gabbroic magma. But we have seen above 

 that Bowen's scheme, based on the gravitational accumulation of 

 crystals and operating with temporary minerals which disappear 

 in the later stages of the crystallization of the magma, meets with 

 several difficulties of theoretical and calculative character. I think, 

 therefore, it is more correct to acknowledge that the problem of 

 the monomineral igneous rocks and in particular, of the anortho- 

 sites, is not yet solved. It were better for this problem to remain 

 an unsolved riddle to stimulate further investigations than to be- 

 come, if unilaterally solved, a possible source of dogmatism. 



PeTROGRAX) 



May, IQ2I 



